Oxygen is known as one of the most important gaseous species for the lifecycle of a human being. Its presence and concentration is of high relevance in many fields, including health care, food industry, security etc. High precision oxygen sensors are therefore much sought-after.
Different molecules absorb electro-magnetic radiation typically in dependence of the frequencies of the radiation. Thus, particular molecules have a unique absorption spectrum versus the used radiation frequency, which allows for a specific absorption determination, and thus measurement of the concentration of the molecules. In the case of oxygen molecules the ground state of the molecule is X3Sg− which can be excited to low-lying excited states of a1Dg, b1Sg+, c1Su−C3Du, B3Su−. The dipole transitions between the ground state to all the low-lying excited states are forbidden, so that the ground state of oxygen molecules shows a weak light absorption in the wavelength range from 300-1200 nm.
Methods for measuring the concentration of ground state oxygen molecules have been described, e.g. by Kroll et al., Appl. Phys. Lett., 51 (18) 1987, 1465-1467 or Philippe and Hanson, Appl. Opt. 32 (30) 1993, 6090-6103. In these methods, typically, the weak band transition of b1Sg+-X3Sg− locating around 760 nm is employed for optical absorption spectroscopy to measure the oxygen molecular concentration. As a light source for the absorption, an IR laser at the central wavelength of 760 nm can be utilized, and a photodiode at the corresponding wavelength may be used for the detection of the laser intensity after the absorption. The laser light may accordingly be introduced into a gaseous environment containing oxygen molecules. While the laser light is transmitted through the environment, photons of the laser light are absorbed by the ground-state oxygen molecules (X3Sg−) by the weak band transition of b1Sg+-X3Sg−. This results in a decrease of the laser intensity. The concentration of the oxygen molecules in the ground state (X3Sg−) can subsequently be determined by comparing the decreased laser intensity to the initial laser intensity or an equivalent control laser intensity in the absence of gaseous molecules. The intensity decrease of the laser light, which can be determined in such a laser absorption sensor, is typically proportional to the length of the laser path (L).
In practical implementations of these laser absorption sensors, it may be advantageous to miniaturize the geometric size of the sensor device. However, by reducing the size of the device, also the length of the laser path (L) decreases, at least if only a single-path laser absorption is used. A possible solution to this problem is the employment of multi-path laser absorption, in which the laser light is reflected multiple times by mirrors until the laser light is detected by a photodiode. The effective length (Leff) of the laser path is then multiplied by the times of the reflection (n), asLeff =n×Lgeo   Formula (1)where Lgeo is the geometric length of the distance between the laser and the mirror(s).
Currently known laser absorption sensors are based on a laser as light source of 760 nm radiation and a photodiode for the detection. They typically use single-path absorption, so that the detection limit becomes larger when the dimension of the sensor becomes smaller. In these cases the requirement of the detection limit may not be fulfilled. In addition, by miniaturizing the laser absorption sensors, e.g. to the range of a few cm, space may become restricted so that it becomes difficult to use separate light sources and photodiodes in a setup.
In consequence, there is a need for the development of an improved absorption spectroscopy device for gas measurement applications in a miniaturized format.